A safety belt generally comprises a metal member attached to one end of a rope and a hook attached to the other end thereof. In a sling for a flexible container, a rope is connected to a metal member attached to a container body. Usually the connection of the rope with the metal member is carried out manually by a process in which a rope end is untwisted to a group of strands which are then fixedly incorporated into the rope body. Since this process requires skill as well as considerable strength, it is difficult to obtain operators therefor nowadays. If a narrow width woven fabric is used in place of a rope, the connection may be easily carried out through a sewing operation, but the handling thereof is inferior to that of a rope due to its width.
In a woven fabric used for a sling requiring a high strength, it is necessary to weave a number of warps into a predetermined width of the fabric, whereby the fabric must be three layerd, or two layered while adding a plurality of reinforcing core yarns. An inspection of eight slings available in the market showed that the average thickness was 4.17 mm and the maximum thickness was 5.2 mm (nylon). An estimate was obtained, from the investigation of these weave structures, that the average breakage strength is 7,820 Kgf and the maximum is 10,680 Kgf (polyester) if a strength utilization ratio is assumed to be 80%. As there is a limit to the number of warps that can be woven into a predetermined width, it is necessary to weave fabrics to be undesirably wide in order that the strength requirement is fulfilled. Of course, specially high-strength yarns, such as aramide fiber yarns, may be used for this requirement, but these are so expensive that they cannot be used for general purposes.
The limitation of the number of warps that can be woven into a predetermined width of a narrow width fabric, is mainly determined by the capacity of the loom on which the fabric is produced. It is the above-mentioned slings that are designed and produced within such a strict limitation of the number of warps and thus these fabrics were produced under an extremely uppermost verge of the limitation of the conventional art. The limitations of loom will be described below.
In a needle-type narrow loom, a weft is picked into a warp shed from one side thereof, and received by a latch needle positioned on the other side so that a knitted selvage is formed. During the formation of the selvage, the weft is first caught by a hook of the latch needle. There is no problem when the weft is received by the hook from a back layer of a multilayered thick fabric, but when it is received from a front layer, the weft is liable to detach from the hook if the weft is positioned above a tip end of the hook. Accordingly, a fabric thickness under which a weaving operation is stably carried out is less than 5 mm in a gray fabric, and less than 4.5 mm after the heat-set has been carried out thereto.
In a rack-and-pinion type narrow loom, the weaving operation can be relatively smoothly continued even if a number of warps are woven, because a shuttle passes through a center of shedding while being gripped. However, there is one drawback therein. That is, it is necessary to increase the lengths of shuttle and shuttle box relative to the dimensions of the weaving window so that the shuttle is retained in the original shuttle box until a rack of the shuttle engages with a pinion of an opposite shuttle box. This results in the lowering of loom rotational speed, and since a wider space is required, the loom is generally designed as narrow as possible provided the shuttle can be safely passed. Therefore, if a shedding motion is even slightly disturbed when an extremely thick belt is woven, the passage of shuttle is obstructed and this causes a machine failure.
In a slide-hook motion-type narrow loom, a slide bar movable both in the right direction and left direction with reference to FIG. 10A, is provided in part of a shuttle race. A vertical groove is provided in the slide bar, in which a hook is movable upward and downward by a cam provided inside a slay. Two holes are bored, respectively in the right and left areas of the bottom wall of the shuttle for receiving the hook therein when the shuttle is in the shuttle box to move the shuttle along by the displacement of the slide bar, while the hook is lowered when the shuttle passes through the weaving window. After the shuttle has passed through the weaving window, the hook returns to the hole in the bottom wall of the shuttle to assist the movement of the shuttle.
In the slide-hook motion-type narrow loom, lower side warps forming a shed are brought into contact with the race and the shuttle runs thereon. When an extremely thick belt is woven in which a warp volume exceeds a certain level, the shuttle cannot clear the same, and becomes liable to float, resulting in unstable running and machine failure.
Further, while all parts in the loom frame, the motor arrangement, the weft picking mechanism, the shedding mechanism such as a dobby or the like, and the take-up device must be constructed to be resistant to the high warp tension, this requirement is not satisfactorily fulfilled by the conventional loom.